Aluminum coated ferrous bodies and processes of making them



United States Patent ALUMINUM COATED FERROUS BODIES AND PROCESSES OF MAKING THEM Marshall G. Whitfield, Garden City, and Victor Sheshunofl, Jericho, N. Y., assignors to Whitfield & Sheshunolf, Incorporated, Garden City, N. Y., a corporation of New York No Drawing. Application August 4, 1951, Serial No. 240,446

Claims. (Cl. 29-497) This invention relates to an improved method of coating ferrous metal articles, and more particularly, to a method for cont-rolling the formation of a bond between ferrous metal bodies and aluminum and its alloys. This application is a continuation-impart of our copending application Serial No. 757,920, filed June 28, 1948, and entitled Coating Ferrous Metal Bodies, since abandoned.

When molten aluminum-containing metal is applied either as a coating or a layer to a ferrous metal surface under proper conditions, the aluminum-containing metal combines substantially at once with the ferrous metal surface. It is also recognized that a ferro-aluminum al- 'loy is formed at the interface between the ferrous body and the layer or coating of the aluminum-containing metal, such alloy bonding the coating to the body.

While, when coating ferrous bodies directly with aluminum or aluminum alloys, the formation of a ferro-alluminum alloy at the interface is necessary to obtain a permanent bond between the ferrous metal surface of the body and the coating or layer of aluminum-containing metal, we have discovered that the presence of this alloy in large amounts impairs the strength of the resulting composite structure, and the control of the thickness of the bonding alloy is critical to' the strength and permanence of the bond. In general, the bonding alloy formed at the interface is brittle in nature, and the strength of the resulting bond, insofar as subsequent hot and cold working operations are concerned, is dependent to a large ex tent on the thickness of such alloy. We have found that for most conditions of rigorous working of the resulting laminate of ferrous metal and aluminum or aluminum alloy, the thickness of the bonding alloy formed at the interface should not exceed about 0.6 mil, and preferably should not be in excess of 0.5 mil. By limiting the thickness of the bonding alloy to not more than 0.5 mil, we have observed that the resulting laminate can successfully withstand heat-treating and rolling operations which cannot be efiicaciously carried out with laminates having a thicker bonding alloy at the interface.

It is a principal object of our invention to provide a method and a ferrous article coated with aluminum or aluminum alloys in accordance with the said method, whereby an excellent bond is obtained together with a minimum of interpenetration of the iron and aluminum at the interface, all to the end of providing a superior coated article and one which may be rigorously worked without loss of the coating.

This and other objects of the invention which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, we accomplish by that procedure and in those articles of which we shall hereinafter set forth some exemplary embodiments.

The coating treatment to which we refer is a hot dip treatment in which the ferrous bodies, either as discrete articles or in strand form, are dipped beneath the surface of a bath of the molten aluminum or aluminum alloy. Such hot .dip procedures take a variety .of forms, and the 2,800,707 Patented July 30, 1957 teachings of this application are pertinent to all of them, from hand dipping procedures through those which involve the successive passage of articles, sheets or the like, through coating pots, to so-called continuous processes of aluminum coating, of which a non-limiting example is the process of the Sendzimir Patent 2,110,893.

The teachings of this application are based on our discovery that the metals molybdenum and tungsten, either alone or in combination with each other, when imposed on ferrous bodies as superifical layers, act as barriers during the hot dip coating of such bodies in molten aluminum or aluminum alloys, to control the interpenetration of the aluminum and the iron of the body. We do not wish to be bound by theory as to the nature of any interface alloy which may form. However, the use of tungsten and molybdenum and combinations of the two as barrier layers, as hereinafter taught, is uniformly effective in preventing the formation of observable interface alloy layers having a greater thickness than about 0.5 to 0.6 mil, and is elfective further in providing aluminum coated ferrous articles having excellent bond and high workability without loss of the aluminum layer.

Previous efforts to control the thickness of the interface alloy in hot dipping. processes centered around either the control of the time and temperature of the dipping operation or the addition of ingredients to the aluminum bath. These efforts produced somewhat improved results but, due to the incomplete solution of the problems involved, an easier and more expeditious control was sought. It is an object of the present invention to provide a means of control such as will leave the operator free to carry on his coating processes with primary attention to economic and manufacturing considerations, and to render it unnecessary .to add metallic constituents to the molten aluminum in an attempt to minimize alloy formation.

lt-is not, however, beyond the scope of our invention to include other metallic constituents in the aluminum bath, and in particular, such metals as silicon and magnesium hitherto used for these purposes. Thus, the term aluminum as used hereinafter, is intended to include not only the commercially pure metal, but also aluminumcontaining alloys, taking into account the presence of alloying ingredients other than aluminum, in which alloys, however, aluminum is the major ingredient.

In carrying out the process of this invention, the ferrous metal body is provided on its surface with a thin coating of molybdenum, tungsten or combinations of the two, whenever it is desired to control the thickness of the bonding alloy formed at the interface between the surface of the body to be coated and the aluminum coating metal. The coating of the control metal determines the depth of the bond alloy, if any, without requiring the extremely accurate time and temperature control heretofore required for that purpose. With the alloy thickness control taught herein, the strength of the bond is greatly augmented, since the bonding alloy is reduced in thickness; and this enables the composite metal article to be worked either hot or cold, or to be subjected to wide temperature fluctuations, or both, without impairment of the bond.

The molybdenum, tungsten or combination of the two metals may be applied to the ferrous bodies in various ways. One of these is by the use of ordinary metallizing or metal spraying methods.

In the formation of sheet-like bodies, it is within the scope of our invention to clad ferrous stock at intermediate gauges with molybdenum or tungsten or alloys thereof and roll the product to thinner gauges as has been done in the cladding of iron or mild steel with other metals.

Electrolytic deposition is also available. By way of example, U. S. Patent 2,512,141 to Ma and Ollinger describes a method of electrodepositing a molybdenum compound on a metallic surface and reducing it in a commercial bright annealing furnace at standard temperatures.

Molybdenum and tungsten and combinations of the .two may be deposited on ferrous bodies by the decomposition of halogen compounds of the metals, by the reduction of reducible metallic compounds, or by the reduction of gaseous mixtures. Again, by way of example, tungsten or molybdenum or both may be coated on the surface of the base metal through plating by pyrolysis from the carbonyls. In such a procedure applicable to iron or mild steel, the article is first preliminarily cleaned. It is then subjected to a hydrogen reduction treatment by heating it to about 500 C. in dry hydrogen after which its temperature is raised to at least about 600 C. in wet hydrogen (5% water vapor), and it is held at a temperature substantially between 600 and 800 C. for about minutes. Next, the hydrogen pressure is reduced to about 0.05 millimeter of mercury and carbonyl is introduced into the treatment furnace to give a carbon monoxide pressure of 0.01 millimeter of mercury. The plating may be continued at about 600 to 800 C. for about 10 minutes. This will provide a coating which is both thin and hard.

The thickness of this coating may be increased at a more rapid rate if desired by reducing the temperature to about 525 C., raising the hydrogen pressure to about 0.14 millimeter of mercury as a maximum, and the carbon monoxide pressure to about the same maximum. Plating may be continued under these circumstances to give any desired thickness of coating which will be found to be a moderately hard coating.

Procedures of this character are described in the publication by Lander and Germer entitled Plating Molybdenum, Tungsten and Chromium by Thermal Decomposition of Their Carbonyls, to be found at pages 648- 692 in vol. 175 of the transactions of the A. I. M. M. E. (1948).

Herein and in the claims which follow, the term molybdenum, tungsten and mixtures of the two is intended to be inclusive of barrier metals comprising these elements and their mixture and other alloying ingredients so long as the molybdenum, tungsten or mixture thereof constitutes the preponderant proportion of the alloy.

The teachings of this application are applicable to the coating with aluminum as above defined of any ferrous articles in the broad sense of that term and inclusive of iron and mild steel as well as the various known alloy steels not excluding those ferrous bodies in which molybdenum or tungsten or both are alloying ingredients.

As has already been pointed out, the coating procedures which we use are inclusive of all of the known hot dipping procedures and all of the temperatures for aluminum baths from the melting points of such baths up to and beyond about 1625 F. In the exemplary continuous coating procedure of the Sendzimir patent to which reference has been made above, it is a usual practice to pass ferrous bodies or strands first through an oxidizing furnace wherein combustible matters are burned from the surfaces of the scale-free bodies and a thin, controlled coating of oxide formed thereon. Then the bodies are passed through a reducing furnace wherein this oxide coating is completely reduced; and while the surfaces of the articles are maintained in the reducing atmosphere, they are led beneath the flux-free surface of a bath of molten aluminum or aluminum alloy. The temperature within the reducing furnace may be high enough to anneal the ferrous bodies and indeed may, if desired, be above the A3 point thereof. While it is possible to combine a step of surface deposition of molybdenum or tungsten with the reducing heat treatment of the Send zimir process, this requires special treatment and in many instances it will be preferred to form the coating of barrier metal prior at'least to the reducing step in a continuous coating process. In either event the barrier metal layer will be present at the time the work piece is subjected to elevated temperatures in the reducingfurnace and under these circumstances there may be a greater or less degree of diffusion of the barrier metal substance into the substance of the iron body. The practice of our invention is inclusive of diffusion of the barrier metal or metals into the base to the extent that such diffusion occurs or progresses in the subjection of the coated base to heat treatments for any purpose in the manufacture of articles suitable for coating with aluminum. In such diffusion incident to heat treatments, there will be a concentration of the barrier metal or metals at or in the surface portions of the ferrous bodies to which they were applied. The effect of minimizing the formation of interface alloy layers is, however, retained in spite of such diffusion so long as the metal of the body at or near the initially coated surfaces has a preponderant content of the barrier metal or metals.

The thickness of the layer of barrier metal or metals initially applied may be widely varied; and we have found a useful effect in the production of workable coated articles arising from use of exceedingly thin continuous layers such as layers of the order of 0.00002 in. Economical considerations, which will vary with different coating procedures, provide, so far as we have been able to ascertain, the only upper limit to the thickness of the initial coatings which may be employed.

Modifications may be made in our invention without departing from the spirit of it. Having thus described our invention in certain exemplary embodiments, what we claim as new and desire to secure by Letters Patent is:

1. A method of forming a high strength bond between a ferrous metal body and a coating of aluminum which comprises coating the surface of the ferrous metal body with a barrier metal chosen from a class consisting of molybdenum, tungsten and mixtures thereof and dipping the ferrous body into a bath of molten aluminum.

2. The process claimed in claim 1 wherein a thin but continuous and imperforate barrier metal layer is initially formed on the surface of the ferrous body by roll cladding.

3. The process claimed in claim 1 wherein a thin but continuous and imperforate barrier metal layer is initially formed on the surf-act of the ferrous body by chemical reduction.

4. The process claimed in claim 1 wherein a thin but continuous and imperforate barrier metal layer is initially formed on the surface of the ferrous body by pyrolytic deposition from a carbonyl.

5. The process claimed in claim 1 wherein a thin but continuous and imperforate barrier metal layer is initially formed in the surface of the ferrous body by electroplat- 6. A method of forming a high strength bond between a ferrous metal body and a coating of aluminum which comprises coating the surface of the ferrous metal body with a barrier metal chosen from a class consisting of molybdenum, tungsten and mixtures thereof and hot dipping the ferrous body into a bath of molten aluminum, the ferrous body between the deposition of said barrier metal on the surfaces thereof and the hot dipping in aluminum being subjected to a heat treatment whereby the barrier metal is caused to diffuse into the surfaces of the said ferrous body to form an interface alloy no greater than 0.6 mil in thickness.

7. The process claimed in claim 1 wherein the ferrous body coated with said layer of barrier metal is subjected to a heat treatment whereby to some extent at least there is brought about a diffusion of the barrier layer into the surfaces of said ferrous body.

8. The process claimed in claim 1 wherein the ferrous body coated with said layer of barrier metal is subjected to a heat treatment whereby to some extent at least there is brought about a diffusion of the barrier layer into the surfaces of said ferrous body, the said heat treatment occurring in a reducing atmosphere and the said body being led beneath the surface of a bath of molten aluminum while still protected by said reducing atmosphere.

9. A ferrous body having a surface coated with aluminum by hot dipping, there being between said body and said coating no more than about 0.6 mil of interface alloy, said coated body being further characterized by the presence in substantial amounts at the said interface of a metal chosen from a class consisting of molybdenum, tungsten and mixtures thereof derived from a coating of such metal applied to the said body prior to the application of the aluminum.

10. A ferrous metal body coated with aluminum and having the characteristics arising from a coating process in which a continuous layer of a barrier metal chosen from a class consisting of molybdenum, tungsten and mixtures thereof is first imposed upon the surface of said body after which said body is dipped in a bath of molten aluminum.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF FORMING A HIGH STRENGTH BOND BETWEEN A FERROUS METAL BODY AND A COATING OF ALUMINUM WHICH COMPRISES COATING THE SURFACE OF THE FERROUS METAL BODY WITH A BARRIER METAL CHOSEN FROM A CLASS CONSISTING OF MOLYBDENUM, TUNGSTEN AND MIXTURES THEREOF AND DIPPING THE FERROUS BODY INTO A BATH OF MOLTEN ALUMINUM. 